Shetland dialect, the northernmost Scots variety, is something of a conundrum. Although most of its features place it at the end of the Northern Scots dialect continuum, some lexical, phonological and structural features resemble characteristics of more southerly Scots dialects; in particular those of the east central counties and the North-East. This essay approaches this problem from the point of view of recent work on new dialect formation, demonstrating that many of the features associated with this phenomenon — koinéisation, focussing, and the founder effect, among others — can be postulated for the development of Shetland dialect. Because this new dialect was formed further back in time than those previously studied, the pattern of development is rather more complex and difficult to describe than for those formed in the 19th century; moreover, the developing Scots dialect was for a lengthy period in contact with its close relative Norn in the islands. Because of the latter’s associations with local identity, indeed, we could see this as a further founder effect. The essay demonstrates that present-day Shetland dialect was formed in the early 19th century from the supraregional koiné of the original 16th and 17th century Scots-speaking settlers and the heavily Norn-influenced Scots of the first and second generations of islanders who no longer had Norn as a mother tongue.